In general, conventional phase change inks are in the solid phase at ambient temperature, but exist in the liquid phase at an elevated operating temperature of an ink jet printing device. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the printing medium, they quickly solidify to form a predetermined pattern of solidified ink dots.
More recently impulse inks or bubble jet inks have been disclosed in U.S. Pat. No. 5,021,802. These inks comprise an aqueous sol-gel medium and coloring agents. The inks are thermally reversible sol-gels and exhibit conventional phase change ink behavior such that they are gels (solids) at ambient temperatures and sols (liquids) at elevated temperatures between about 40.degree.-100.degree. C.
An inherent problem associated with aqueous inks employed in drop-on-demand color printers is "feathering" of ink drops after placement onto a substrate such as paper. Feathering is the undesirable nonuniform dispersal or wicking of ink drops along the paper fibers after the inks are placed on the print medium. Feathering may cause intercolor bleeding, poor resolution, and image degradation adversely affecting the print quality. Attempts to resolve this problem include the use of cosolvents to reduce drying time, the use of latex-based inks, the addition of zwitterionic surfactants, the use of pigmented inks, and the use of solid inks. Although solid inks tend not to feather when printed, their implementation significantly increases the cost and complexity of the equipment needed to print such inks.